Benzo(a)pyrene is metabolized either to nontoxic water soluble conjugates or to the ultimate carcinogen, BP-7, 8-diol-9, 10-epoxide. The direction of BP metabolism might be dependent on the presence of specific predominant forms of enzymes involved in the BP metabolism of individuals exposed to chemical carcinogens. Monoclonal antibodies (MAB) are specific to antigenic determinants. Therefore, Balb/c female mice were immunized with microsomes obtained from the placenta of women who smoke heavily and fractions of human blood monocytes induced with benzo(a)anthracene. The primed spleen cells were fused by polyethylene glycol with myeloma cells and grown in selective media to isolate hybridomas. 21/73 and 87/205 independent hybrid clones tested are positive for mouse immunoglobulin production to monocyte fractions and human placenta microsomes. The immunoglobulins produced by hybrids were types of IgGl and IgM and inhibited aryl hydrocarbon hydroxylase activities of human placental microsomes by 15 - 45%. MAB reacting with human cytochrome P-450 would be very useful for the testing of cytochrome P-450 levels of individuals exposed to different environmental carcinogens.